This invention relates to a new sustained ignition secondary combustion unit for the secondary combustion of smoke products from primary fuel burning, either as a built in or retrofitted component of a furnace or stove or as a self contained separate unit. The invention is particularly applicable for woodburning furnaces and stoves.
Efficient burning of wood fuel requires sufficient oxygen and maintenance of sustained high temperatures throughout the combustion zone. Conventional wood stoves and furnaces are generally inefficient because of the failure to maintain sufficient temperature for thorough oxidation & combustion of the volatile gases driven off by primary fuel burning. Even more recent wood stoves which seek to incorporate secondary combustion of the volatile gaseous products from primary wood burning are unable to mix enough secondary air for total combustion of the gases without cooling the mixture below the ignition point. As a result up to fifty percent of the energy content of the wood fuel escapes through the chimney in the form of these volatile gases. Thus, conventional wood stoves which operate at moderate domestic scale outputs suffer from inefficient combustion because of either insufficient temperature or insufficient secondary air.
Furthermore, it is important that the functions of primary wood fuel burning and secondary combustion of volatile gases be handled sequentially. Preferably this is accomplished by at least partially separated primary and secondary combustion chambers. Conventional wood stoves with a single combustion chamber which admit more air intending to increase efficiency merely burn the wood faster increasing the primary wood fuel burning rate. This may not necessarily increase the combustion of volatile gases driven off by the primary burn. Thus, without a secondary combustion chamber efficient and complete combustion is difficult to attain without burning the wood fuel at an excessive rate. Where a secondary combustion chamber is added such prior art wood stoves or furnaces suffer the defect of insufficient temperature or insufficient secondary air as discussed above.